1. Field of the Invention
The present invention relates generally to the field of liquid measuring devices, and more particularly but not by way of limitation, to an improved calibration container which is inexpensive to manufacture yet sufficiently rugged to retain its accuracy during field service.
2. Discussion of Prior Art
Virtually every commercial liquid, including petroleum products, herbicides, insecticides, paints and the like, is measured by volume, either by filling a container of known capacity, or by transfer through a flow meter. When a flow meter is used to measure the quantity of a single liquid of known and relatively constant physical characteristics, such as that of a gasoline pump at a retail service station, the flow meter can be calibrated with the expectation that the accuracy of the indicated volume transferred through the meter will not vary over a long period of time.
However, in many other applications, such as the dispensing and mixing of agricultural herbicides, insecticides and the like, the flow rate indicated by a flow meter can be adversely affected by many factors, including varying viscosities of metered liquids and the solids content of the liquid. Dispensing and mixing operations of such liquids frequently take place in an open field or other location removed from a laboratory or factory environment where calibrations are usually initially made. Thus, there is a need for an inexpensive, sturdy, corrosion resistant container of known volumetric capacity which is suitable for use in calibrating a flow meter under field conditions.
Volumetric measuring of liquids has been addressed by numerous inventors, such as the earthenware liquid measure taught by Young in U.S. Pat. No. 258,845, who provided a device for calibrating total volumetric capacity by the inclusion of a removable supplemental ceramic piece fastened near the inside bottom of a container. Shapcott, in U.S. Pat. No. 3,880,012, disclosed a graduated measuring flask having a single volume mark with a permanent dark area immediately below the volume mark to enable the meniscus to be viewed more clearly against a white background.
Hudspeth, in U.S. Pat. No. 4,033,190 provided a device for accurately measuring large volumes of liquids, the device having a predetermined overflow level together with an overflow saver vessel for catching and preserving any liquid above the desired amount. Further, the Hudspeth patent teaches the addition of a displacement structure which is used to permit accurate changes in the measured volumes within the primary vessel. The size and complexity of the Hudspeth device renders it unsuitable for utilization beyond a factory or chemical processing environment.
Other calibration devices of general interest include those found in Nafis U.S. Pat. No. 685,850; Paoli U.S. Pat. No. 3,461,728; and Hach U.S. Pat. No. 3,905,768. These patents show various volumetric measuring devices which can be used in a controlled environment.
Probably the most widely accepted solution adopted for field calibration of liquid flow meters which are used in service involving corrosive fluids is the provision of specially constructed containers dedicated to the measurement of a specific fluid. For example, commercially available containers are frequently used for field calibration of in-line flow meters. One such prior art measuring container which is presently available for such service is that manufactured by the Ellisco Company of Philadelphia, Pa. The Ellisco test container is made of a heavy duty terneplate steel and has a transparent viewing window in its neck portion for the observation of the liquid meniscus. A graduated plate is adjustably secured in juxtaposition to the viewing window. The Ellisco container is fabricated by precision metal forming and welding techniques which, while generally acceptable, is expensive and is susceptible to miscalibration if dented or otherwise misshaped during agricultural related usage. Further, the transparent viewing window can be broken or marred, negating the utility of the containing for calibrating purposes.
The cost of fabricating prior art calibration containers, such as the Ellisco container, would lead one to look to the possibility of molding such containers with synthetic polymeric materials. However, the accuracy required of the calibration containers has heretofore weighted material selection in favor of metallic materials even at substantial higher manufacturing and material costs.
None of the prior art containers known to the present inventor features portability, ease of manufacture and sufficient ruggedness to withstand the rigors of use at a field site, while also being relatively inexpensive to manufacture.